Cardiac arrhythmias have traditionally been segregated, for treatment by drug therapy or a cardiac rhythm management device, into bradyarrhythmias and tachyarrhythmias. Patients having certain bradyarrhythmias that are not responsive to drug therapy are often possible candidates for an implantable pacemaker device to provide electrical pacing stimuli to manage their irregular and abnormally slow cardiac rhythms. Patients having certain tachyarrhythmias that are not responsive to drug therapy are often possible candidates for an implantable cardioverter-defibrillator device to provide anti-tachyarrhythmia therapy, such as low-energy cardioversion countershock therapy or higher-energy defibrillation countershock therapy, to manage their irregular and abnormally fast cardiac rhythms. Other cardiac rhythm management devices incorporate anti-tachycardia pacing for converting a tachyarrhythmia into a normal cardiac rhythm.
A particular concern in cardioverter-defibrillators is avoiding the delivery of inappropriate countershock therapy in the absence of heart activity indicative of a need for such therapy. Because of the significant energy levels typically used in cardioversion and defibrillation countershocks, patients typically experience a significant degree of fright and discomfort if such therapy is delivered inappropriately. This may, in turn, put the patient at risk of other injuries arising from whatever activity the patient is engaged in when the inappropriate therapy is received. Repeated instances of inappropriate countershock therapy may also deplete the power source of the implantable cardioverter-defibrillator, reducing the implanted longevity over which the device is effective at providing anti-tachyarrhythmia therapy. Moreover, the inappropriate delivery of countershock therapy increases the difficulty for the physician in properly diagnosing and treating the cardiac patient.
Countershock therapy may itself pose certain adverse side-effects. A countershock may temporarily degrade cardiac function, resulting in a period of ischemia. A countershock may increase myocardial irritability, leaving the patient in a state of increased susceptibility to induction of tachyarrhythmias, including fibrillation. Actual post-countershock arrhythmias may also result. Because of the risks inherent in countershock therapy, its inappropriate delivery should be avoided whenever possible. Other anti-tachyarrhythmia therapies also have certain inherent risks. For example, anti-tachycardia pacing therapy includes a risk of inducing and accelerating certain tachyarrhythmias, including life-threatening ventricular fibrillation. Because of the risks arising from such anti-tachyarrhythmia therapy, its inappropriate delivery should also be avoided whenever possible.
Certain patients have cardiac arrhythmias requiring both pacing therapy for bradyarrhythmic episodes, and anti-tachyarrhythmia therapy for tachyarrhythmic episodes. One problem in that is particularly troublesome in cardiac rhythm management devices providing both pacing and anti-tachyarrhythmia therapy, is that delivery of the pacing therapy may increase the risk of delivering an inappropriate countershock under certain conditions. Because of the above-described risks inherent in delivering inappropriate anti-tachyarrhythmia therapy, there is a need in the art for a cardiac rhythm management device, such as an implantable cardioverter-defibrillator, that provides both pacing and anti-tachyarrhythmia therapies without exacerbating such risks by the delivery of pacing or other bradyarrhythmia therapy.